alice writes M1 on node-1
bob writes M2 on node-2
  M1=0b986f63-1329-43ef-bf98-f120841fbfe1 M2=8c1735cd-0da9-40a9-803e-f3d4fec8782e
settle 5s for pre-link replication
alice links M1 -> M2 with relation=related_to
  link POST returned HTTP 201
settle 8s for link fanout
charlie queries links of M1 on node-3
  charlie sees M1->M2 link: 1 (expected >=1)